Firstly check you have the WinXP firewall disabled...
now check the usernames and passwords on the machines.
for example...
Win2K box:
user: administrator
pass: blah
WinXP box: user
pass: wibble
You need to set up another user on the Win2K box (you never need to use it, its just so the WinXP machine can connect) using the WinXP info. So in the example above, you would add a user profile to Win2K called "user" with the password "wibble".
Also, check both machines are in the same workgroup - WinXP tends not to complain so much about workgroups (well, it hasnt in my experience anyway lol), but Win2K does.
Also check the IP addresses are in the same range and subnet - although they probably will be because they need to be in the same range as the router in order to get online, it might be worth checking it. Check that you're using static IP addressing rather than DHCP - if you're only using 2 machines, its easier with static because you know what IP each machine is on (I tend to always use static IP addressing as its easier to work out which machine is which).
Can you ping the other machines? i.e. ping Win2K from WinXP and vice versa?
Try those things out and post back how you get on.